don't subscribe to channels if fetching stats fail

priv/frontend/src/components/App.js

@@ -25,7 +25,14 @@ export default class App extends Component {
   }
 
   subscribeToChannels () {
+    if (this.state.metrics == null) {
+      // fetching state failed, bail
+      log('not going to subscribe to realtime updates due to fetch failure')
+      return
+    }
+
     const { graph } = this.state.metrics
+
     const channels = Object.keys(graph).reduce(
       (channels, name) => [...channels, `latency:${name}`, `status:${name}`],
       []